At the present time, one of the most widely used anodic materials in the electrolytic production of chlorine and alkali, chlorates and hypochlorites is titanium having an active coating of mixed oxides of ruthenium and titanium with a molar ratio of RuO.sub.2 :TiO.sub.2 =30:70. These electrodes are known as "DSA"--dimensionally stable anodes. These anodes are resistant towards corrosion, selective and exhibit high catalytic activity. Indeed, the stationary rate of their corrosion (q) under conditions close to those for chlorine electrolysis e.g. 300 g/l NaCl, pH 4, 87.degree. C., i=2kA/m.sup.2, is 2.6.times.10.sup.-8 g/(cm.sup.2 h) of metallic ruthenium while the concentration of oxygen in chlorine gas is v=2.4 vol %. Both values increase when the acidity is decreased, and at pH 5 they comprise q=6.2.times.10.sup.-8 g/(cm.sup.2 h) and v=4.7 vol %. The increase of the dissolution rate of ruthenium of such DSA anodes with the increase of pH limits the application of these materials in the production of chlorine and alkali by membrane technology. Occurring defects in membranes lead to alkalification of electrolyte at the electrode surface and destruction of the coating. Anodes based on mixed oxides of iridium, ruthenium and titanium (IrO.sub.2, RuO.sub.2, TiO.sub.2) are characterized by higher than DSA corrosion resistance. (U.S. Pat. No. 3,948,751, issued 1976 to G. Bianchi et al and U.S. Pat. No. 4,564,434, issued 1986 to Busse-Machukas et al.)
Anodes based on mixed oxides IrO.sub.2 and TiO.sub.2 (30 mol. % of IrO.sub.2) are disclosed in U.S. Pat. No. 3,632,498, to Beer issued 1972. However, these electrodes did not find wide application due to low catalytic activity in chlorine evolution reaction. This drawback was successfully corrected by means of simultaneous introduction of iridium and ruthenium oxides into the coating,--aforesaid U.S. Pat. Nos. 3,948,751 and 4,564,434. It should be noted that the concentration of RuO.sub.2 in those electrodes was usually higher or at least comparable with the concentration of IrO.sub.2. For example, in U.S. Pat. No. 3,948,751, the molar ratio of IrO.sub.2 to RuO.sub.2 is IrO.sub.2 :RuO.sub.2 =0.5:1, while the ratio TiO.sub.2 :(IrO.sub.2 +RuO.sub.2)=(3.8 to 7.8):1. In U.S. Pat. No. 4,564,434, the concentration ratio of IrO.sub.2 :RuO.sub.2 was varied in the range of (0.75 to 3):1 while TiO.sub.2 :(IrO.sub.2 +RuO.sub.2)=(1 to 3):1. The potential of these electrodes under conditions of chlorine and alkali production e.g. 280 g/l NaCl, 87.degree. C., pH 3-3.5 and under conditions for sodium chlorate production e.g. 400 g/l NaClO.sub.3, 100 g/l NaCl, 2.5 g/l Na.sub.2 Cr.sub.2 O.sub.7, pH 7, T 80.degree. C. was close to that used with aforesaid DSA anodes and at i=2kA/m.sup.2 was in the range of 1.32-1.33 V and 1.4-1.43 V vs NHE. It is noteworthy that even when the ratio of the components in the coating of 15 mol. % IrO.sub.2 +15 mol. % RuO.sub.2 +70 mol. % TiO.sub.2 is believed to be optimum, these electrodes are better as far as corrosion resistance is concerned than DSA electrodes only by a factor of 1.5-2 times.
It has been established that the electrodes described in U.S. Pat. No. 4,564,434 display about half of the corrosion resistance of those with individual IrO.sub.2 coating (U.S.S.R. Authors Certificate No. 1,611,989--Belova et al.). The latter however, lack the catalytic activity of those anode of U.S. Pat. No. 4,564,434 in the chlorine evolution reaction (see Table 1).
There are different ways known to prevent formation of a passive layer on a valve metal support. For example, U.S. Pat. No. 4,469,581, issued 1984 to Asano et al. discloses alloying with multivalent metals and creating random crystalline structures. U.S. Pat. No. 4,331,528, issued 1982 to Beer H. B. et al. discloses forming non stoichiometric oxides of passive metals on the anode substrate; and by formation of relatively dense films from an oxide of a support metal with incorporated metals of the platinum group utilized either as a metal or a compound.
In the latter work, it was demonstrated that the most strongly protective properties were shown by dense oxide films of titanium, containing oxides and chlorides of iridium and/or rhodium. Even a loading of 0.5-0.6 g of noble metal per m.sup.2 of geometric surface prolonged the life time of electrodes with highly porous active coating of the DSA-type about 10 times.
There is therefore, a need to increase the reliability of protection of anode metallic supports from oxidation and from the formation of blocking layers, especially under conditions of significant oxygen evolution.